In recent years, electronic circuit boards that have an electronic circuit pattern formed on an insulating substrate, such as a plastic substrate (possibly film), ceramic substrate or metallic substrate coated with plastic have been developed. Means for configuring an electronic circuit by soldering electronic pats, such as IC devices, semiconductor chips, resistors and condensers, on the wiring face have been widely adopted.
In this case, for the purpose of bonding the lead terminal of an electronic part to the prescribed part of a circuit pattern, the procedure generally followed comprises preparatorily forming a thin solder layer on the surface of an electrically conductive circuit electrode exposed on the electronic circuit board, printing solder paste or flux, positioning and mounting a prescribed electronic part and subsequently causing the thin solder layer alone or the thin solder layer and the solder paste together to reflow, thereby completing a bond with the solder.
Recently, the trend of electronic products toward miniaturization has been urging their electronic circuits to use fine pitches. Parts of fine pitches, such as LSIs of the QFP (Quad Flat Package) type and the CSPs (Chip Size Packages) having a pitch of 0.3 mm and FCs (Flip Chips) having a pitch of 0.15 mm, have come to be copiously mounted in small areas. For this reason, the electronic circuit boards have come to need fine solder circuit patterns in conformity with the fine pitches.
For the purpose of forming soldered circuits with solder film on an electronic circuit board, the plating method, the HAL (Hot Air Leveler) method or the method which consists in printing a paste of solder powder and causing the paste to reflow is available. The method for producing a soldered circuit by the plating technique encounters difficulty in forming the solder layer in a necessary thickness and the HAL method and the method utilizing the print of solder paste encounter difficulty in attaining conformity with fine pitch patterns.
As a means for forming a soldered circuit without necessitating troublesome operations, such as alignment of circuit patterns, a method which comprises imparting tackiness to the surface of the electrically conductive circuit electrode of an electronic circuit board by causing a tackifier compound to react with the surface, attaching solder powder to the tackified part and subsequently heating the electronic circuit board, thereby fusing the solder and forming a bonded circuit has been disclosed (refer, for example, to JP-A HEI 7-7244).
Owing to the method disclosed in the prior art, it has become possible to provide an electronic circuit board of high reliability by having a fine solder circuit pattern formed by a simple procedure. Since this method attaches solder powder in a dry state to the electronic circuit board, it inevitably suffers the powder to be attached as by static electricity to an irrelevant part other than the necessary portion and even to be excessively attached to the exposed metallic surface of the electronic circuit board. Thus, the desirability of developing a technique for efficiently removing the excess solder powder afterward has been finding acceptance. When the dry procedure is used at all, however, the drift of powder and the like ensue and obstruct use of fine pitches in the electronic circuit board. Besides, the excessively attached solder powder somewhat undergoes oxidation during the course of the dry treatment and entails persistently a problem regarding the reuse of recovered solder powder. This problem gains in prominence particularly when the solder powder to be used has a very minute particle size.
This invention, in a method for producing an electronic circuit board by treating the exposed metallic surface (the surface of an electronically conductive circuit electrode) on an electronic circuit board with a tackifier compound, thereby imparting tackiness thereto, attaching solder powder to the tackified part, and then heating the electronic circuit board, thereby melting the solder and forming a soldered circuit, is aimed at providing a method for the attachment of solder powder which is capable of realizing as fine a circuit pattern as possible, a method for producing a solder-attached electronic circuit board by a procedure of causing the attached solder powder to reflow in accordance with the method mentioned above, an electronic circuit board possessing a fine circuit pattern and exhibiting high reliability, an electronic circuit board having mounted thereon an electronic part capable of realizing high reliability and high mounting density, and a method for the reuse of solder powder inducing no appreciable degradation during the course of attachment of solder powder.
The present inventor, after making diligent effort and study with a view to solving the problems mentioned above, has reached this invention. To be specific, this invention has solved the problems mentioned above by developing the following items.